1. Field of the Invention
The present invention relates to a triac, and more specifically to a triac having similar sensitivities in the four triggering quadrants.
2. Discussion of Related Art
Vertical triacs, that is, triacs comprising, on a so-called rear surface, a first main electrode (A2) and, on the opposite or front surface, a second main electrode (A1) and a gate electrode (G), are considered herein. Generally, a triac comprises, side-by-side, PNPN and NPNP structures forming two head-to-tail thyristors. A portion of the front surface is dedicated to the trigger or gate structure and enables, when a voltage is applied between the gate electrode and the main front surface electrode, to trigger the thyristor, which is properly biased for the applied voltage.
Currently, triacs are formed within a substantially square contour, the PNPN and NPNP thyristors substantially taking up half of the useful surface area, and a small portion of this surface area being dedicated to the triggering structure, which is generally arranged in a corner of the square.
Four triggering quadrants are generally distinguished according to the voltages present on the main electrodes and on the gate electrode. Calling A2 the main rear surface electrode and A1 the main front surface electrode, and considering that main electrode A1 which is used as a reference for the gate is at a zero voltage, the four quadrants, Q1, Q2, Q3, Q4 are defined as follows
A1A2GQ10>0>0Q20>0<0Q30<0<0Q40<0>0
FIGS. 1A, 1B, and 1C respectively show a top view, a cross-section view along line B-B, and a cross-section view along line C-C of FIG. 1A of a conventional corner gate triac. Reference will be made hereafter to all these drawings.
The structure is built from a lightly-doped N-type semiconductor substrate 1. It comprises a P-type well 3 on the front surface side and a P-type layer 5 on the rear surface side. A first NPNPP+ thyristor between main electrode A1 and main electrode A2 comprises a heavily-doped N-type region 7 formed in P well 3 and regions 3, 1, and 5, as well as a more-heavily doped P-type region 9 on the rear surface side. The second P+PNPN thyristor between main electrode A1 and main electrode A2 comprises a P+ region 11 formed in the upper portion of P well 3 in contact with electrode A1 and, on the rear surface side, a heavily-doped N-type region 13 in contact with electrode A2. These two thyristors generally have substantially equal surface areas.
The triggering structure is formed in P well 3. It comprises a heavily-doped N-type region 15 generally having a specific shape of the type shown in FIG. 1A, surrounded with a heavily-doped P-type region 17. The gate metallization is in contact with these two regions 15 and 17. It should be noted, by observing FIGS. 1A and 1B, that a P-type well region 18, which is not overdoped, exists between gate regions 15, 17 and main regions 7, 11 of the front surface electrode. In FIG. 1A, the contour of main electrode A1 is indicated by dotted lines and the contour of gate electrode G is also indicated by dotted lines. A heavily-doped N-type region 19 is formed under the gate area, on the rear surface side.
An N+-type channel stop layer 20 has been formed at the periphery of the front surface of the triac. Different types of peripheries may be used, especially according to whether the triac is of planar or mesa type. These peripheries will not be detailed herein since they are well known by those skilled in the art.
As known, in order for the triggering to occur favorably, many conditions should be complied with, and many gate topologies, as well as many shapes of N region 19 placed on the rear surface in front of the gate, have been devised. Similarly, it should be noted that, generally, the doping of well 3 (and thus of layer 5) is selected according to compromises between the sensitivity and the immunity to parasitic triggerings of the triac. P+ layers 11, 17, and 9, which are necessary to provide an ohmic contact with the electrodes, will have optimized shapes to improve the triac sensitivity.
Still, those skilled in the art have to make a compromise. If the auxiliary thyristor is made too sensitive, the triac has strong dV/dt triggering risks, that is, it risks triggering when the voltage between its main terminals varies abruptly while no gate voltage is applied.
As an inevitable result, in all corner-gate structures of the type shown in FIG. 1, current IGT which should flow between the gate and main electrode A1 to trigger the thyristor is much greater in quadrant Q4 than in the other quadrants and the dV/dt parasitic triggering characteristic for all operating modes of the triac is not optimal.
Many other alternative embodiments of triacs are known by those skilled in the art. In particular, generally, so-called emitter short-circuit holes are provided in main N+ front surface and N+ rear surface regions 7 and 13. This means that each of the N+ layers is locally interrupted so that the P region in which it is formed makes flush at the level of these interrupts.
The triggering mode of this structure in mode Q4 (A2<0, G>0) is the following. When a positive voltage is applied to the gate with respect to main electrode A1, a current flows from gate metallization G into P region 17 and travels to P+ region 11. The flowing of this current, especially through region 18 along N+ region 7, creates a voltage drop greater than 0.6 V and makes the junction between P well 3 and N+ region 7 conductive. This results in a carrier generation under the gate region and this strong injection carrier generation modifies the naturally blocking behavior of the junction between P well 3 and substrate 1. The auxiliary thyristor comprising regions 17-3-1-5-19 turns on and its turning-on causes the turning-on of main thyristor 11-3-1-5-13.
FIG. 1A shows the area in which the triggering starts in quadrant Q4 (reference Q4). The starting area in quadrants Q1 and Q2 (reference Q1,Q2) and the starting area in quadrant Q3 (reference Q3) have been similarly shown. It should be noted that the triggering areas are clearly separate, which enables to better understand the difficulty of balancing the triggering sensitivities in the various quadrants.